The present invention relates to a diffractive grating member and an optical communication module.
In the conventional diffractive grating member for an optical pickup device that reads signals from two types of media of CD and DVD, a laser beam with wavelength 785 nm used for reading a CD and a laser beam with wavelength 655 nm used for reading a DVD are targets, and a laser beam with a wavelength for CD and a laser beam with a wavelength for DVD are made to be of the same optical axis, and signals are detected by a single light-receiving element. Therefore, an optical path difference between adjoining diffractive surfaces is made to be the same as a certain wavelength, and a step-shaped grating on which six surfaces are formed is used.
Further, in the optical communication field, well-known is an optical communication module in which two wavelengths are used respectively for a receiving side and a transmitting side, and which separates the light fluxes with two wavelengths to couple light fluxes from a semiconductor laser to an optical fiber and to couple light fluxes from the optical fiber to the light-receiving element. In the following Patent Document 1, there is used a step-shaped grating wherein a wavelength of a semiconductor laser is made to be 1310 nm and a wavelength from an optical fiber is made to be 1550 nm, while, an optical path difference between adjoining diffractive surfaces is made to be the same as a certain wavelength, and a step-shaped grating on which three surfaces are formed is used.
(Patent Document 1) TOKUKAI No. 2003-344715
Though the step-shaped grating can separate light fluxes with two wavelengths, it is difficult to separate light fluxes with three or more wavelengths. In the optical communication field, in particular, when attaining 3-wavelength optical communication module wherein light from a semiconductor laser is coupled to an optical fiber stably without being affected by a slight fluctuation of wavelength, and light respectively with two wavelengths from an optical fiber are separated to be guided to different light-receiving elements, it has been difficult for the conventional step-shaped grating to separate light from an optical fiber sufficiently.